This invention relates generally to information recording systems and, more particularly, to servo apparatus for moving an information storage medium, such as a disc, cylinder, or tape, relative to a device for recovering information therefrom.
An example of a servo apparatus of this particular type is disclosed in a co-pending and commonly assigned application for U.S. patent, Ser. No. 890,670, filed Mar. 27, 1978 and now abandoned in the name of R. Dakin et al. and entitled "Video Disc Player". In that application, a system is described having a spindle motor for rotating a disc-shaped record carrier and a tachometer coupled to the spindle motor for producing a signal representative of its angular velocity. The servo apparatus is operable to lock the phase of the tachometer signal onto the phase of a locally generated reference signal.
The above system has not proven completely satisfactory, however, because the angular velocity of the spindle motor does not necessarily correlate precisely with the angular velocity of the record carrier relative to the information recovery device. Eccentricities in the shape of the record carrier and vibrations brought about by the relative movement cause undesired variations in the relative velocity of the particular information track being read. Moreover, the signal produced by the tachometer does not necessarily correlate precisely with the angular velocity of the motor, because of eccentricities and other defects in the tachometer, itself. Still another drawback to a system of this type arises because the cost of the tachometer and the manufacturing costs for mounting it are relatively high.
An example of an alternative servo apparatus of this particular type is disclosed in U.S. Pat. No. 3,350,503, issued in the name of D. P. Gregg and entitled "Electron Beam Recording and Reproducing System". The technique disclosed in this patent avoids the need for a tachometer, but in doing so, introduces other drawbacks. The apparatus disclosed includes a gun for directing an electron beam at a magnetic tape to read information recorded thereon, and means for controllably deflecting the beam longitudinally along the tape to compensate for variations in its relative velocity. The beam is deflected in accordance with a comparison of a local reference signal having a predetermined frequency and a pilot signal recovered from the tape. By this technique, the velocity of the actual tape relative to the electron beam can be precisely controlled, and the effects of tape speed variations, i.e., "flutter" and "wow", can be thereby minimized.
Although this latter servo technique has proven effective in some applications, it cannot by itself yield a constant velocity for the record medium in applications where the record medium has a plurality of parallel information tracks and the device for recovering the information recorded thereon is not always aligned with an individual track or is made to traverse the record medium from one track to another. Such non-critical positioning can occur, for example, when the information recovery device is not properly focused and is simultaneously reproducing information recorded on several adjacent tracks or when the device is not properly aligned with an individual track.
It will be appreciated from the foregoing that there is still a need for apparatus for precisely controlling the velocity of a record medium relative to a device for recovering information therefrom, which is operable even when the recovery device is not critically positioned with respect to an individual information track or is traversing the record medium from one track to another, and which can do so without undue complexity or cost. The present invention fulfills this need.